1. Field of the Invention
The present invention relates generally to a device and method for measuring turbulence in high-speed flows, and more particularly to a microsensor thin-film probe capable of measuring turbulence in hypersonic flows.
2. Description of the Related Art
Turbulence measurements in high-speed flows have historically been obtained by hot-wire anemometry. However, high stagnation temperatures, high dynamic pressures, and flow contaminants severely limit the life of hot-wire elements in hypersonic flow. Nonintrusive measurement techniques such as laser-Doppler velocimetry and particle-image velocimetry also are limited when applied to hypersonic flow. In particular, data-rate limitations and difficulties in flow seeding present the most significant obstacles to their accurate application in high-speed flows. An alternative to conventional hot-wire anemometry is hot-film anemometry, with a thin metallic film deposited along the stagnation line of a rigid, dielectric substrate, thus increasing mechanical strength.
Hot-film probes incorporating various combinations of materials and construction techniques have displayed excellent durability and moderate frequency response characteristics in the few high-speed and high temperature flows in which they have been tested.
Ling and Hubbard introduced the thin-film probe as a resistance-temperature transducer to measure turbulent fluctuations in flowfields in which hot-wires could not survive (Ling, S. C. and Hubbard, P. G. 1956 "The Hot-Film Anemometer: A New Device for Fluid Mechanics Research", J. Aeronaut. Sci. 23, 890). This probe consisted of a thin layer of platinum fused to a glass substrate. The main body of the probe consisted of a 4.0 mm diameter Pyrex rod with two 32-gauge platinum lead wires (2.0 mm apart) embedded within the core. The rod was ground down into a 8.degree. wedge, tipped by a 30.degree. wedge. Fused on the wedge tip was a thin platinum film sensor (1.0 mm.times.0.2 mm) with a nominal cold resistance of 20.OMEGA.. The ends of the sensor were attached to the exposed lead wires by thick platinum overplatings. It was tested at high temperatures (1100.degree. F.) without detectable changes in thermoelectric properties. Experiments indicated a frequency response of 100 kHz at a flow velocity of 1000 ft/sec.
Later, Seiner used commercial hot-film probes to investigate high-speed, cold jet flows (Mach 0.5-2.0), (Seiner, J. M. 1983 "The Wedge Hot-Film Anemometer in Supersonic Flow", NASA Technical Paper 2134). The probe consisted of a thin film of nickel sputtered on a 4.degree. semivetrex wedge of quartz substrate. A protective coating of quartz (0.5-2.0 .mu.m) was sputtered over the nickel (1.0 mm.times.0.2 mm) for electrical isolation and protection from particles. A maximum frequency response of 130 kHz for the 1:1 balanced CTA bridge was realized via the square-wave injection test, and was found to be inversely proportional to the thickness of the protective coating.
More recently, Demetriades and Anders presented a report on the ongoing development of a constant-current probe for use in high-temperature/hypervelocity flows (Demetriades, A. and Anders, S. G. 1990 "Characteristics of Hot-Film Anemometers for Use in Hypersonic Flows", AIAA Journal 28, 2003). The design consists of a platinum sensor (0.5 mm.times.1.8 mm) painted along the stagnation line of a wedge-shaped glaze bead positioned at the tip of a twin-bore alumina tube (10.0 cm.times.0.25 cm). Results from temperature endurance testing (temperature cycling to 1400.degree. F.) demonstrated the excellent electrical stability characteristics of the probe. These probes were also run for hours in high-temperature, high dynamic-pressure environments (Mach 8.0, 800.degree. F. and 20.7 kPa) without failure, further confirming the durability characteristics. Presently, there is no experimental data available from Demetriades documenting the frequency response characteristics of this probe. However, painted sensors contain cross-sectional non-uniformities which lead to "hot-spots" and sensor failures. In addition, the large sensor surface area used by Seiner and Demetriades limits the spatial resolution of the measurement technique.
The frequency response characteristics of the existing hot-film probes are inadequate to resolve the full turbulent spectrum for hypersonic flows. The "dual swept-surface" wedge designs used by Seiner are a poor approximation of stagnation-line heat-transfer, since the sensor extends 0.1 mm away from the tip on both sides of the wedge. In addition to this fundamental problem, Seiner found that the probe displayed poor directional behavior and was Mach-number sensitive. Seiner hypothesized that these problems were associated with the 40.degree. wedge geometry and recommended examination of a larger semi-vertex angle or a wedge fitted with a rounded nose. Finally, this design is unacceptable for measurements in wallbounded shear flows, as shocks emanating from the bottom of the probe would disturb the flow.